The present invention relates generally to a temperature detecting device, and more particularly, to a temperature detecting device employing a thermistor as a temperature detecting element.
In the case of controlling the temperature of various devices within a broad temperature range, it is necessary to provide a temperature detecting device. A temperature detecting device of the conventional type is, shown in FIG. 1. However, in this device, it is necessary to provide a number of voltage comparing circuits. Therefore, such detecting circuits occupy a large area when formed by integrating them as an IC together with other circuits having other functions.
Another temperature detecting circuit is shown in FIG. 2 and this is a more recent circuit construction having one voltage comparing circuit, however a plurality of such temperature detecting circuits are employed in the conventional type. However, the circuit construction shown in FIG. 2 is not suitable for completely forming the same by C-MOS IC.
In the circuitry of FIG. 2, the input operational voltage range of a voltage comparing circuit constructed by C-MOS is limited by the threshold voltage of the MOS-transistor and the change in voltage "Vt" as a function of changes in temperature is shown in FIG. 3 according to the non-linearity of a thermistor as a temperature detecting element. As can be seen in FIG. 3, the change in voltage ".DELTA.Vt" of the voltage "Vt" for a unit change in temperature (1.degree. C.) becomes remarkably smaller when the temperature becomes higher. Therefore, the accuracy of the temperature reference voltage for comparing the voltage "Vt" is necessarily high. Further, it is necessary to accurately determine the resistance dividing ratio for generating the reference voltage and to make smaller the ON-resistance of the switching element. If an MOS-transistor is employed as the switching element, it is necessary to fabricate a transistor having a broad channel width for resuming the ON-resistance thereof, and accordingly the area occupied by the transistor on the IC-chip becomes larger.
The present invention aims to eliminate the above noted drawbacks and, therefore, an object of the present invention is to provide a temperature detecting circuit having a high detection accuracy without occupying a large area of the IC.